Liquid batteries, whether flow or no-flow, are known in the art and work on the same principles as solid batteries, except the electrolyte is liquid. Such batteries are comprised of electrochemical cells which are based on reduction-oxidation chemistry. Oxidation occurs on the anode side of the cell and reduction on the cathode side. The solvents used in electrochemical cells are varied. In many circumstances, aqueous solutions are used on both sides of an electrochemical cell with each side (cathode side and anode side) in contact with an electrode (i.e., the cathode and anode respectively). The electrodes of the two-half cells are placed in electrical contact to allow for current to flow. To maintain charge balance, an electrochemical cell must also allow for the passage of ions. In elementary batteries, this is done with a salt bridge separating the cathode solution from the anode solution. The bridge prevents mixing of the two solutions. If the solutions were to mix, the half-cells could be destroyed by direct chemical reaction.
As with most batteries, flow batteries typically deploy a membrane separating the anode electrolyte from the cathode electrolyte. The role of the membrane is to allow for the exchange of ions but without mixing of the electrolyte solutions and thus the membrane preserves the electrochemical cell. In addition, for flow batteries, the electrolyte is continuously replaced (thus the terminology “flow”). Membranes, however, are a major weakness in batteries generally and in flow batteries in particular because they tend to degrade with time (especially in the presence of strong bases and acids) and are costly.
Membraneless flow batteries have been reported in the literature, but they too suffer from significant drawbacks. Such membraneless batteries are often termed “laminar” flow batteries because they rely on laminar flow to maintain separation of the analyte and the catholyte. A conventional laminar flow battery does not have a membrane and operates because of the slow rate of mixing of the two fluids in the laminar regime. However, mixing does occur, resulting in waste and, if allowed to progress, the mixing will short-circuit the battery. Membraneless systems have been proposed in which the electrolytes are selected on the basis of their pH. However, such systems can produce a precipitate at the interface between the electrolytes. Accordingly, there remains a need for further contributions in this area of technology.